The present invention relates to a device and method for determining the occurrence of a rare short circuit. More particularly, the present invention relates to a rare short circuit determining device and a rare short circuit determining method that detects abnormal current flowing through an electric circuit of an automobile.
U.S. Pat. No. 4,023,264 describes a blade fuse installed in a fuse box of an automobile. Blade fuses, which are often used in electric circuits of automobiles, have a slow-burn characteristic. Due to the slow-burn characteristic, blade fuses are not melted by momentary overcurrents but are melted by continuous overcurrents that flow for a certain period of time.
Blade fuses normally melt and break when a dead short circuit occurs but do not break when a rare short circuit occurs. A dead short circuit causes a large amount of current to continuously flow in an electric circuit. A rare short circuit causes current to flow intermittently and within a short period of time when, for example, vibrations cause the electric wiring of an automobile to contact the body of the automobile. When a rare short circuit current continuously flows through the electric wiring of the automobile, for example, the electric wiring may be heated.
Japanese Unexamined Patent Publication Nos. 61-191231 and 7-131925 describe methods for determining the occurrence of rare short circuits. However, these methods focus only on current values and do not accurately prevent electric wires from being heated.
The method described in Publication No. 7-131925 determines an abnormality level based on a cumulative value of overcurrent per unit time. Thus, rare short circuits are not accurately recognized in the method. The abnormality determination is delayed or advanced depending on how long the unit time is. Thus, if the abnormality determination is performed too early, a rush current, which is typically produced when using a lamp as the load circuit, may be determined as being abnormal.
One of the inventors of the present invention disclosed a method for determining a rare short circuit in Japanese Unexamined Patent Publication No. 2001-45651 (Japanese Patent Application No. 11-21553). In the method, the occurrence of a rare short circuit is determined based on one of at least four characteristic values. The characteristic values refer to an abnormal load current exceeding a predetermined current threshold value, the time during which abnormal current continuously flows, an ON-duty ratio, which is the ratio of the time when abnormal current is flowing relative to a predetermined time, and the number of times the abnormal load current exceeds the predetermined current threshold value. This accurately determines the occurrence of a rare short circuit. However, the determining method is complicated. Thus, there is a need to determine the occurrence of a rare short circuit in a simplified manner.